Home-  Sailboat-  Submarines-  ROVs-  Metal Working-  Other Stuff -  About Us

Why Build a Sub
   Submarine 101
   Wet Sub
   Planing Wet Sub
   Dry Ambient Sub
   Submarine Yacht
Design Evolution
Cutting Aluminum
Building the Hull
Kort Nozzles
Battery & Chargers
Engine & Jet Drive
   Engine Tune-Up
   Hull Cooler
   Wet Manifold
   Engine Mounts
   Engine Box
   Jet Pump Valve
Helm Controls
Ballast Sled
Landing Gear
Trim Tanks
Wiring Harnes
Tow Truck
Rules of Thumb



Trim Tanks

Submarines use trim or ballast tanks to control both buoyancy and trim of the boat. Ballast tanks are simply containers that can store air or water in order to increase or decrease lift. If the nose is too heavy then air is injected into a forward trim tank to displace some of the water and the nose rises. 

There are two types of  trim tanks or ballast tanks; hard and soft.  Hard tanks are fully enclosed tanks. Once the required amount of air is inside the tank, then the vents are closed and the volume of air remains the same even if the submarine ascends or descends.

Submarines use trim or ballast tanks to control both buoyancy and trim of the boat. Ballast tanks are simply containers that can store air or water in order to increase or decrease lift. If the nose is too heavy then air is injected into a forward trim tank to displace some of the water and the nose rises. 

There are two types of  trim tanks or ballast tanks; hard and soft.  Hard tanks are fully enclosed tanks. Once the required amount of air is inside the tank, then the vents are closed and the volume of air remains the same even if the submarine ascends or descends.

Soft tanks are normally a submarines main ballast tanks and are not used for trim. Soft tanks are open at bottom.  When air is injected into a soft tank is it normally done so with the intension of completely blowing the tank or emptying it of water. Because soft tanks are open, the air that is injected into the tank will compress if the sub descends and it will expand if the sub ascends. So if a little air is injected the sub will normally start to ascend, and as the sub ascends the pressure in the tank decreases and the air expands causing more water to be displaced and the sub to rise quicker.  For this reason soft tanks are normally not used for trim tanks.


(1) Soft tanks and air supply.
Red - Ambient air flow lines
Green - Ballast tank bulk heads
Blue - Ballast vents
Purple - Variable vent pipes

(2) Front view

(3) Side view

Likewise the deeper you go the more the air in a soft tank is compressed.  In fact if you start out at the surface with the container completely filled with air, and you somehow dive to 33 feet, the air in the container will have been compressed to half of its original volume.  Or in other words the container will now be half full of water.  That is the bad part about soft ballast, because now your sub has taken on more water and you will begin to sink easier and faster, further compressing the air. This will continue until you reach the bottom unless you add more air to the ballast tank. 

The sub however has two high pressure air cylinders stored in the hull that provide a about 3 cubic feet of air each minute.  This is fresh breathing air but it also replenishes the air in the soft tanks that may have been lost due to compression. The soft tanks also have a vent pipe that can be raised or lowered to adjust the amount of air that is maintained in the tank when the sub ascends. (1) (2) (3) The original design called for three soft trim tanks.  One at the rear and two more at the front, just behind the cabin wall, but the two forward tanks were later combined into one much larger tank in order to increase the freeboard when sitting on the surface before the hull has been pumped empty.

Air first flows into the cabin for breathing. The air supply is always just above the pressure of the outside water so it not only keeps the air from becoming stale, but it also keeps the water outside the hull from flooding in the bottom divers hatch when it is opened, and it keeps the cabin from being crushed from the pressure of the surrounding water.  The air then flows from the cabin through tubing back to additional equipment that needs to be pressure compensated against the water pressure and into the engine compartment.  From near the bottom of the engine compartment the air will bubble up into a divider.  At the divider it will flow to the forward trim tank if the nose is down, or it will bubble up into the rear trim tank if the nose is up. Once the trim tanks are full to the level of the their vent pipes the excess air will be vented overboard.

(1) Linier actuator for adjusting
the vent pipe in the trim tank.

(2) Linier actuator disassembled.

(3) Actuator detail.

(4) Actuator position control

(5) Forward trim tank vent and
linier actuator control.

Vent Pipes

(1) The vent pipes in each trim tank can be adjusted so that the tanks hold more or less air inside. Originally I had planned to use control rods linking the vent pipes to the cabin but I latter switched to using linier actuators that can have three preset stop points set.  When the end on the vent pipe is rotated upward, then air in the tank will is released and replaced with water that floods in from the bottom.  When the pipe is rotated down, then the air bubbling up into the tank will slowly displace the water in the tank until the water level in the tank descends enough to allow the air to again flow out of the vent pipe. 

To monitor the ballast tanks, each of the vent pipes will have a float switch just above the lower end of the pipe.  This will indicate that the vent is under water and the ballast displacement is not in sync with the vent position.  This switch might latter be integrated with a solenoid valve attached to the high pressure air supply which would then add the required air.  The ballast air flow lines are also sloped so that a water trap is not formed even when the boat is pitched down or up.  Check valves down stream from the cabin and again behind the engine compartment prevent the flow from reversing and flooding the engine compartment when the divers hatches are opened. Small manual purge valves will be allow excess water to be purged from the cabin once the divers hatches are closed.

(2) (3) The two actuators and their controllers came from and altogether cost $280. (4) These actuators have potentiometers built-in and the controller has 3 buttons and it can be used to set 3 stop points for the actuators by simply holding the specific button for 5 seconds once the actuator reaches the desired location.  This will make it easier to return the boat to a trimmed condition once the initial trim is established. The control unit for is separate from the actuator and the the switch so it will be easy to isolate in a 1ATM box to protect the circuits from pressure.

On closer inspection this actuator will work very well. All of the pressure sensitive electronics are confined to the control unit. The switch board only consist of micro switches, LED's and a few resisters.  The actuator will allow ambient air to flow easily thought the unit following the wiring pathways and the piston has a sufficient wiper seal.  The entire unit can easily be made air tight with an coating of epoxy.  Extending the control cable is needed but that is a simple task.

(5) The forward vent and actuator is now installed. A short 1 1/4" aluminum pipe is welded in place to vent air thought a top corner of the trim tank.  A section of automotive radiator hose connects to that, and then to a section of  1 1/4" PVC that extends to the opposite corner.  The actuator connects between the side of the hull and the joint between the hose and PVC pipe. The PVC pipe extends between two arcs of aluminum angle that stabilize it and guide it as it moves up and down.  I tried using the hose as a hinge or pivot point but the pipe would not consistently return to the exact position so I added a brace beside the hose that in connected bracket that attaches the actuator to the vent pipe and hinged to the hull where the hose connects to the aluminum pipe.




(1) Air flows from the engine compartment through a check
valve and to the air divider.

(2) Testing that air will open the
check valve  going to the air
divider before it escapes through
the snorkel air intake.

Air Divider

(1) Before the actuators can move the vent pipe in order to vent or trap more air in the trim tanks the air must reach the trim tanks.  Pneumatic air lines will carry air from the cabin to each of the pieces of equipment that need to be compensated for pressure and finally to the engine compartment.  From the engine compartment the excess air will escape through a vent that is about 5 inches above the point at which the air will have sufficient pressure to vent thought the snorkel.  The vent hose from the engine compartment curves to the port side and thru a check valve and into the air divider mounted on the side of the hull.  The air divider either sends the bubbles forward or releases them into the aft trim tank depending on the pitch of the boat. The divider is just an aluminum box mounted level with the keel, and as air escapes through the vent line it discharge into the side of the divider.  Since the divider is full of water the air will form bubbles. If the front of the boat is slightly raised then the bubbles will move forward along the top of the box and be release to the side where they will bubble up into the rear trim tank.  If the front of the boat has a slight down angle then air will flow to the back of the box and up into an 1 1/4 inch pipe.  This pipe will be coupled to a PVC pipe which will carry the air 14 feet forward to the front trim tank. The PVC pipe has 1 inch holes drilled in the bottom side in order to allow the water to easily move aside and allow the air bubbles to pass.  Without these holes the pipe would act as a lift pipe and carry water along as well.  That is not a big issue but not necessary either. [Oops - Strike the holes in the pipe the bottom of the trim tank is lower that the holes so it would never be able to be filled went surfacing because the air would escape out the holes.] The bubble supply pipe has a 10 degree uphill slope in relation to the keel as it runs forward.  That means that air will be delivered to the forward trim tank as long as the any down angle on the bow is maintained at less than 10 degrees.  If that angle is exceeded trimming the boat will require either releasing trim from the rear tank, or moving the ballast sled toward the rear or a combination of the two.  I guess hitting the bottom might do the trick too

The check valve is necessary because when surfacing the engine will be started while the hull is still full of water. The check valve will prevent water from being drawn back into the engine compartment when the engine air compartment quickly converts from ambient pressure to a vacuum created by the engine drawing air down the snorkel.

(2) While putting things together I changed my plan and just vented the bilge pumps to the air divider. I thought I had killed two birds with one stone until I realized that air would actually escape back through the snorkel before the air pressure was great enough to push water out thought the bilge pumps. I went back to the original plan of venting the engine compartment form a port that is 5 inches above the opening to the snorkel.  Then I got to wondering if 5 inches of water pressure was enough to open the check valve. I sure seemed like enough pressure but a experience was warranted as changing the system later on would be a royal pain. So I taped the vent line into a bucket, drilled another hole 3 inches down, hopped in the outdoor tub with the rig along with an air line from the shop and tried it out.  Sure enough it worked like a charm.  I'd guess that the check valve opened up with only about 1 inch of head pressure.  It's nice to know something is going to work, especially on the day after the oil cooler blew out.

Air Supply

Human Air Requirements stated by the NOAA Dive Manual is 2 scfm (standard cubic feet per minute) for each person at rest and
4 scfm for each person not at rest. The SportSub's flow rate is 90 cu ft of air per hour for 3 people which is only .5 scfm per person.  

High pressure air will be supplied by two 300 cubic foot compressed air tanks that are stored in each side of the hull.  These tanks are available from welder supply stores and are refilled with breathable air for less than scuba tanks.  Actually you don't even wait, you just exchange your empty tanks for full tanks.  You also don't ever have to bother with a hydro-test, because the supplier will take care of it. The only problem is fitting a scuba regulator to these tanks because they come with a "CGA 346" type outlet, and not the common DIN or Yoke valves used on scuba tanks or even the "CGA 580" that is found on Argon tanks or the "CGA-320" found on C02 tanks.  That eliminates the possibly of using Argon regulators without an adapter. The plan is to use a high pressure, small diameter hydraulic line connected to the tank and deliver high pressure air into the cabin where it connects to a MIG welding regulator.  These regulators have a gauge to display the pressure remaining in the tank and a regulator capable of providing a slow and steady flow of air around 3 cubic feet per minute.  There are two separate air cylinders, supply hoses and regulators so a failure of one system still leaves the second system available to surface the boat. 

Putting It Together

(1) Various air supply plans.

(2) Two tables of assorted parts.
Many more are on their way.

Boy howdy; this is one I underestimated. I figured I would run a couple of high pressure lines to regulators and feed that into the cabin and then put in a couple of lines that bleed that back to the equipment and the engine compartment. (1) (2) I've now gone through several designs and I am running out of  bench space for the parts that are accumulating. It took 3 nights just to find the parts I needed, and you can spend money on pneumatic parts faster than you can at strip club.

First thing I did was overcomplicate it because I did not see an easy want to mount my two welding gas regulators inside the cabin. I think I got that problem solved by putting my regulators in the lath and converting them into panel mount regulators so that only the control handle will protrude into the cabin. All of the air lines will be under the seats.  Pneumatic control stuff is only good for about 120 psi so I'll use the regulators to keep the psi down.  Not much psi will be needed as the main goal is to flow about 3 to 4 cubic feet of air per minute through the cabin and then back to the equipment.

I did add a couple of 4-way pneumatic control valves that interrupt the flow of air to the cabin and redirect it directly to the forward and aft trim tanks respectively.  This provides an emergency burst of air to the trim tanks should the rate of compression exceed the rate at which new air is flowing into the tanks.

Pneumadyne, has a great website for learning about and buying pneumatic parts.

The high pressure hoses are actually hydraulic hoses. It use to be that you had to have these custom made because the ends needed to be crimped on with a special tool.  However you can now find reusable fittings.  I found to be a good source.  I'm using 1/4" SAE 100R2AT hydraulic hose and 1/4" hose x 1/4" NPTF Male fittings for all of the high pressure lines.

SAE 100R2AT hydraulic hose interchanges with: Weatherhead H425, Aeroquip FC212, and Parker 301 or BXX

The tank is a female CGA 346, that goes to and adapter with 1/4" Female NPTF.  The other end is the regulator and with the CGA 580 nipple removed it provides 1/4" Female NPTF. And out from the regulator with another 1/4" Female NPTF and to another adapter for a pony bottle pressure gauge with another 1/4" Female NPTF.  So all of the hoses are 1/4" with reusable 1/4" NPTF Male fittings.

Emergency Surface System

Most submersibles have drop weights in order to get back to the surface when things have gone wrong.  From an ambient submarine getting out and swimming is much easier than it is from a 1ATM, but just the same it would be nice to get back to the surface along with the sub. We don't have drop weights, but we do have two really big trim tanks, and the forward tank alone can surface the sub. The primary dangers are malfunction of the trim tank vent actuators, failure of the 12 volt system that the trim tank actuators use, a pitch down angle greater than 10 degrees that prohibits air from flowing to the forward trim tank, failure of the air supply to both trim tanks due to leaks.  The high pressure air system is already redundant, so it does not make my top concerns list.

(1) Check valve and push-pull
cable on aft trim tank.

(2) Check valve on the forward

(3) Housing over aft valve
because it faces forward.

(4) Aft view of the check valve
with the housing in place.
(5) Left - At the bottom of the
photo is one of the controls for
push-pull cable.

(6) Above - Valves that vent air
directly to trim tanks with the
regulator and air gauge.

The primary failure can be solved by adding the ability to manually seal the trim tank vents and supply air directly to the tanks. It took some time to figure this out but I am very pleased with the result.

(1) (2) We added a swing gate check valve where each trim tank vents to the open water. The trick is that the check valve is turned so that it prevents air from exiting the tank and then the gate is held open by a push-pull cable that is operated from the cabin.  In an emergency we simply pull the cables so that the check valves close and seal the trim tanks.

(2) We added a hand rail to the cabin top that also provides a conduit for the push pull cable which operates the forward check valve.

(3) (4) The aft check valve faces forward to accommodate the push pull cable.  Since it presented a possible snag point on the hull, a housing was added over the valve.

(5) The two push-pull cables enter the cabin at the sides, just above the seat. When pulled, the push-pull cable retracts from the check valve, allowing the gate to close and seal the tank.  The cable is easily capable of opening the check valve gate even if the tank is full of air, so it is possible to use the vents as means to re-attain a desired trim condition without the need to utilize the trim tank vent actuators.

(6) The second part of the system is two pneumatic push button valves on the low pressure line that supplies the cabin and ambient system.  These are in the line just after the air has flowed through the regulators from the high pressure system. Pressing the forward button will interrupt the flow to the cabin and vent the air directly into the forward trim tank. The aft button will do the same for the aft tank. 

These buttons can also be used if the sub's buoyancy is too negative and the decent rate is compressing the air in the trim tanks faster that the air is being supplied by the normal air exhausted from the cabin.

Links -- Great resource for leaning and buying pneumatic parts.  --Good selection and pricing for hydraulic/high pressure air line parts.
Linier Actuator Suppliers: - Model subs site with lots of ballast and control concepts expanded and demonstrated.  Air Flow Meter MMA-10,  30-300 scfh, 5/16 OD Connector, $17  Pneumatic Fittings: Tube is 1/4 inch, Threads are 1/8 inch pipe.